Structure sensitivity of ammonia

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

Structure Sensitivity of Ammonia Synthesis An ultrahigh vacuum chamber equipped with a high-pressure cell was developed to study the ammonia synthe-... 

Table 1 Surface structure sensitivity of ammonia synthesis...

Fig. 6. The surface structure sensitivity of ammonia synthesis catalyzed by single-crystal iron under high-pressure conditions.

The reader who has followed the arguments up to this point may ask the following questions what is the average typical structure of the activated catalyst and what is the difference between metallic iron and the activated catalyst If the reader is practically minded, he/she may further ask what is the use of this information or, less provocatively, given the well-established structure sensitivity of ammonia synthesis over single crystals, is there a similar relationship between the structure of the technical material and its catalytic usefulness An artist s impression of the main structure features of the technical activated catalyst has been developed, to respond to these questions. 

Figure 4.2. The rate of ammonia synthesis over the five iron surfaces studied exhibiting the structure sensitivity of ammonia synthesis/...

Figure 4.4. The structure sensitivity of ammonia synthesis over rhenium single-crystal surfaces/ Schematics of the atomic structure of each surface is given above each bar.

Table 3.6. Structure sensitivity of ammonia synthesis on Re single crystals...

The structure sensitivity of catalytic reactions is often striking. As an example, Spencer et al. [5] demonstrated a factor of over 400 difference between the activity of the Fe (111) surface and that of the Fe (110) surface (the former being the more active one) for ammonia synthesis. This type of investigation, while of course not predictive in nature, has the promise... 

We have seen how studies on the various faces of Fe and Ni have contributed to the understanding of the structure sensitivity of the ammonia synthesis and of the hydrogenolysis of alkanes. Clearly, the effect of crystal faces, steps, and kinks on other reactions should be pursued. In some cases, kinetics can be measured at vacuum conditions, permitting the use of transient methods. The example of CO oxidation is striking (315). 

Figure 2.6. Top figure Surface structure sensitivity of iron catalysed ammonia synthesis.

Table 2.10 Change of a real rate with crystal-lograpic orientation for a structure-sensitive reaction (ammonia synthesis on iron single crystal) ...

Fig. 39. The remarkable surface structure sensitivity of the iron-catalyzed, ammonia synthesis...

A large amount of Mossbauer work has been devoted to in situ surface characterization of iron catalyst. These experiments were carried out, however, using small particles of iron oxide supported on inert carriers. These materials represent a different class of catalysts to the industrial ammonia synthesis catalyst. The Mossbauer studies which has helped to evaluate the concept of structure sensitivity in ammonia synthesis will therefore not be discussed here. 

The (111) surface can be considered a rough surface, since it exposes second-and third-layer atoms to reactant gases in contrast to the (110) surface which only exposes first-layer atoms. Work functions are related to the roughness of a surface and it is useful to quantify the corrugation of a plane in this way. The work functions of all the iron faces are not currently available but they are for tungsten, another bcc metal which also shows structure sensitivity for ammonia decomposi-tion. The order of decreasing work function (4>) is as follows < no> < 211 > 0100 > 0210 Open faces, like the (111) surface, have lower work functions... 

In this paper we will discuss the results that we have obtained studying two important processes, the synthesis of ammonia over Fe and Re catalysts and the hydrodesulfurization of thiophene on Mo. These examples illustrate two problems in catalysis to which our methods are particularly sensitive. The first is the effect of the structure of the catalyst on reaction rates. The sensitivity of the... 

Structure Sensitivity over Pe. Table II presents the rates of ammonia synthesis over each of the low Miller index planes of Pe. 

Structure Sensitivity over Re. As in the case of the Fe catalysts the rate of ammonia synthesis varies greatly over Re single crystal surfaces of different orientations. This phenomenon has been studied over the (0001), (loTo), (1120) and 0121) planes in a 3 1 Hp/N mixture at a total pressure of 20 atm. and a temperature or 870 K. Under these conditions these surfaces catalyze the reaction with relative rates of 1 94 920 2820 respectively, showing a range of activities even greater than that observed on Fe. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

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